The present invention relates to a semiconductor integrated circuit (IC) device. More particularly, the invention relates to protective circuits for protecting an insulated gate type transistor at an input in the internal circuit of a semiconductor IC against a high voltage accidently applied to the input terminal and to such a protective circuit having a silicon-on-sapphire (SOS) structure in which desired circuit elements are fabricated in a silicon island on an insulating substrate.
A protective circuit is generally provided in fabricating a metal-oxide-semiconductor (MOS) IC utilizing a silicon substrate in order to prevent an excessively high voltage from being applied accidently from an input terminal to an input gate of an MOS transistor of an internal circuit. The protective circuits are provided because the insulation layer such as, for example, the silicon dioxide layer in series with the gate, is very thin, so that if too large a voltage is applied from the input terminal, as experienced with electrostatic discharge from handling on a dry day, the oxide layer ruptures due to excessive electrostatic field stress. In order to prevent this, the protective circuit includes a resistor of a resistance value in the order of 1 k.OMEGA. connected in series and a protective transistor connected between the gate of the protected internal transistor and ground. The protective transistor is also fabricated with one MOS transistor, but a field oxide layer of approximately 7000 .ANG. thick is used as the insulating layer and its gate is fabricated with an aluminum layer even for the silicon gate MOS IC's. Thus, the threshold voltage of the protective transistor is approximately ten times larger than that of the internal transistor, of which the threshold voltage is usually about 0.8 V. Under normal operating conditions, the protective transistor is OFF, but when an abnormal voltage is applied to the input terminal, it turns ON causing a voltage drop at the resistor and permitting charges to be passed to ground. This maintains the voltage at the gate of the internal transistor at a low value and protects said transistor. In other words, the input voltage at the input terminal is not applied directly to the internal transistor by virtue of the resistor, but is applied to the protective transistor first.
It has also been proposed to replace the protective transistor with a diode. For convenience, N-channel type MOS IC's are considered here. When a negative voltage is applied, the diode permits current to flow to ground. When a positive voltage, which is higher8c than the higher voltage of the MOS device, is applied, current is also permitted to flow to ground. However, the performance of the diode is not as effective as that of the protective transistor.
In a SOS IC, a silicon semiconductor layer is grown on an insulating sapphire substrate and the field oxide layer surrounding an active region of the silicon semiconductor layer is etched away, or partially removed by etching, and oxidized to form an insulating layer of silicon dioxide to confine the active region to a silicon island. A gate insulating layer and a gate electrode of polycrystalline silicon are fabricated centrally on the silicon island. Utilizing the gate electrode as a mask, source and drain regions are formed by diffusion of impurities or by ion-implantation of impurities, and an MOS transistor is formed. There is practically no problem in using this MOS transistor as the internal transistor. However, it is not suitable to use the MOS transistor as the protective transistor, because the gate insulating layer is thin and has a low voltage withstanding ability, so that said layer itself is likely to be ruptured. It is thus desired, as is the case with a bulk MOS IC, to utilize a thick field oxide layer surrounding the silicon island in order to form a transistor similar to the protective transistor. However, in the SOS structure, there is an insulating substrate, and not a silicon semiconductor, immediately below the field oxide layer, so that such a transistor cannot be fabricated.
Another circuit is in use as a protective circuit in an SOS MOS IC, because the field oxide layer cannot be used for the gate oxide layer of the protective transistor and it is difficult to make a thick gate oxide layer in addition to the thin gate oxide layer in the internal circuits. Such circuit contains, in addition to the series resistor, two silicon gate MOS transistors, both P-channel type. A first of the two transistors is connected between the gate of the protected transistor and the positive power source, and the second of the two transistors is connected between the gate and ground. The gate is connected through the resistor to the input terminal. The gate of the first transistor is connected to the power source, and the gate of the second transistor is connected to the gate of the protected transistor. Therefore, under normal conditions, that is, when the voltage at the input terminal is equal to the level between the power source and ground, the first and second transistors are both OFF. When the voltage Vi at the input terminal has a value as EQU Vi&gt;&gt;Vcc&gt;0 or Vi&lt;&lt;0,
or more particularly, when Vi exceeds the value EQU Vcc+.vertline.Vth3.vertline.
where Vth3 is the threshold voltage of the first transistor, said transistor turns ON to pass current from the input terminal to the power source. When the voltage of input terminal falls below- .vertline.Vth4.vertline., where Vth4 is the threshold voltage of the second transistor, said second transistor turns ON and causes a current flow between the input terminal and ground. The first and second transistors may be N-channel type, in which case the gate of the first transistor is connected to the gate of the protected transistor, and the gate of the second transistor is connected to ground. The gate of the protected transistor of the internal circuit is thus protected. However, the gate insulating layer of either the first or second transistor is as thin as that of the protected transistor, so that when an excessively high voltage is applied to the input terminal, the insulator between the drain and the gate of either the first or second transistor is very likely to be ruptured.
The principal object of the invention is to provide a protective circuit for protecting an insulated gate transistor at an input in the internal circuit of semiconductor integrated circuits against a high voltage.
An object of the invention is to solve problems experienced with an SOS structure, the field oxide of which cannot be utilized.
Another object of the invention is to fabricate a protective transistor without complicating the processes for the manufacture of such protective transistor.